1. Field of the Invention
The invention relates to a darning width adjusting device for a sewing machine such as an overlock machine.
2. Description of the Related Art
A conventional over-edge sewing machine includes an upper knife and a lower knife, both cutting a workpiece cloth at upstream side of a stitch point in a cloth feeding direction, a lower knife holder provided movably in a direction orthogonal to the cloth feeding direction and holding the lower knife, a thread-sliding piece supported on the lower knife holder and adjusting a darning width by engaging a tip portion of the thread-sliding piece with a thread in the vicinity of the stitch point, and a positioning device positioning the lower knife in the direction orthogonal to the cloth feeding direction (for example, see JP-A-2005-000401).
However, in the conventional over-edge sewing machine, since the thread-sliding piece is supported on the lower knife holder, when the lower knife holder is moved outward to assure a wide cloth cutting width, an upper looper moving from underside of a throat plate toward a sewing needle and the thread-sliding piece interfere with each other (for example, see FIG. 6). Thus, there is an inconvenience that the outward moving amount of the lower knife is limited.
As a result, when over-edge sewing is carried out on an elastic cloth for example, a width from the stitch point to a cutting portion of a workpiece is insufficient so that a thread reeling piece cannot follow the expansion and contraction of the cloth (e.g., the cloth after being cut contracts and causes thread redundancy). Consequently, there is a problem that the darning cannot be suitably aligned with the cloth end.
In order to overcome the above problem, a thread tensioner can be operated to adjust a tension of the thread, thereby dealing with a change in the darning width. However, this leads to a serious problem that an adjusting operation is troublesome so that considerable labor is required for an operator and an operating efficiency is lowered,